This invention is directed at a process for separating a feed into a distillate and a bottoms which utilizes less energy than previous designs. More specifically, the subject design is directed at an improved fractionation system utilizing a compression means to reduce the energy requirements of the system as compared with other fractionation systems.
The fractionation of a feed into a distillate and a bottoms is well-known in the art. The fractionation system normally includes a fractionation or distillation zone, a reboiler for reheating the bottoms and returning the bottoms to the distillation zone and a condenser for condensing the distillate from the distillation zone. Typically the reheated bottoms is returned to the column for at least partial vaporization, and the unvaporized portion is removed. The distillate normally is at least partially condensed. A fraction is removed from the system, while the remainder is returned to the distillation zone as reflux. This intermixing of liquid and vapor is necessary for good separation of the feed into the desired distillate and bottoms. Fractionation systems frequently consume large amounts of energy for reboiling the bottoms and large amounts of cooling water for cooling and condensing the distillate. With the increasing cost of energy, efforts are being made to utilize waste heat sources for supplying energy to fractionation systems. Where waste heat sources are not available, other methods of decreasing the net energy consumption of a fractionation system are being considered. It is known in the art to utilize compressors in conjunction with distillation systems. U.S. Pat. No. 2,577,701 utilizes compressors for increasing the pressure of several streams in a fractionation system. U.S. Pat. No. 2,509,136 discloses a system in which the distillate from a fractionation column is compressed and passed in indirect contact with the bottoms to transfer heat to the bottoms. The distillate then is split into two streams, a reflux stream returned to the first column and a second stream which serves as a feed to a second column. A flow controller is inserted to assure constant reflux to the first column. The speed of the turbine is varied to vary the distillate feed rate to the second tower as the composition changes. This results in variation in the reboiling rate and hence a variation in the fractionation efficiency. U.S. Pat. No. 4,023,946 discloses an elaborate fractionation system in which a closed fluid system may be alternatively compressed and expanded to provide reboiling and condensation for the fractionation system. This system does not provide means for maintaining the distillate quality in a relatively simple, reliable manner. U.S. Pat. No. 4,137,129 describes a process in which the bottoms liquid stream from the fractionation column is flashed to cool the liquid which is then used to condense the distillate from the column. The flashed vapor from the bottoms is compressed and used to reboil the contents of the column. This system also does not provide means for regulating the distillate characteristics in a simple, energy efficient manner.
One method for increasing the heat available in the distillate for reboiling the bottoms utilizing compressors is to provide additional preheat to the overheads from the tower to the compressor. There are, however, practical limitations to the addition of preheat to the overheads subsequently passed through a compressor. One limit is the maximum allowable discharge temperature for the compressor and another is the temperature of the available heat source. If the heat source is at a very high temperature it may be more advantageous to use the heat directly to reboil the tower.
Accordingly it is desirable to provide a process which is energy efficient and reliable and applicable where there is no source available at a very high temperature.
It is also desirable to provide a process which permits relatively close regulation of the fractionation operation without the use of elaborate control systems.
It is further desirable to provide a process which permits a heat source to be utilized in a process having a compressor without exceeding the maximum allowable working pressure of the compressor.
The subject invention, although useful when there is a heat source available at a very high temperature, is more particularly applicable where the available heat source is at a relatively low temperature and where the difference in boiling points of the distillate and bottoms is relatively small.